Studies of mutation by radiation in bacteria are proposed. A specific mutagenic process producing GC to AT transitions will be investigated preferentially. This will be accomplished by extensive analysis of revertant bacteria for the presence of specific nonsense suppressors. The proposed procedures will employ nonsense defective bacteriophage and will allow analysis of several hundred individual mutant colonies at all points of interest in mutation experiments. Three examples of GC to AT transition will be considered to establish the characteristics of this transition at three distinct target sites. Experiments regarding nutrient broth enhancement, mutation frequency decline, mutation expression and the development of resistance to photoreactivation will be undertaken with these different examples of a specific mutation event. A general explanation of the data will be constructed and related to a working hypothesis which describes cytosine-containing pyrimidine dimers as mutagenic lesions. The involvement of certain aspects of DNA repair will be suggested by these experiments and will be tested explicitly by biochemical studies of DNA repair metabolism concomitant with post-irradiation incubations affecting mutation. These studies will define a specific mutation process at the molecular level and will indicate both positive and negative roles of DNA repair in this process; a new assay for cellular mutation will be developed.